utils: only look for gems in GEM_HOME.
utils: only look for gems in GEM_HOME.

This avoids issues where a gem is installed but cannot be found.

Fixes #3355.
Fixes #3760.

reitermarkus/brew,ilovezfs/brew,vitorgalvao/brew,sjackman/homebrew,reelsense/brew,hanxue/linuxbrew,reitermarkus/brew,DomT4/brew,MikeMcQuaid/brew,JCount/brew,mahori/brew,claui/brew,nandub/brew,claui/brew,ilovezfs/brew,Linuxbrew/brew,Homebrew/brew,nandub/brew,DomT4/brew,EricFromCanada/brew,reelsense/brew,JCount/brew,EricFromCanada/brew,Homebrew/brew,Linuxbrew/brew,DomT4/brew,hanxue/linuxbrew,vitorgalvao/brew,reitermarkus/brew,alyssais/brew,sjackman/homebrew,maxim-belkin/brew,Linuxbrew/brew,konqui/brew,nandub/brew,mgrimes/brew,mgrimes/brew,MikeMcQuaid/brew,alyssais/brew,vitorgalvao/brew,mahori/brew,sjackman/homebrew,konqui/brew,MikeMcQuaid/brew,mahori/brew,Homebrew/brew,konqui/brew,sjackman/homebrew,alyssais/brew,vitorgalvao/brew,maxim-belkin/brew,claui/brew,hanxue/linuxbrew,Linuxbrew/brew,mahori/brew,ilovezfs/brew,konqui/brew,claui/brew,EricFromCanada/brew,reitermarkus/brew,nandub/brew,reelsense/brew,EricFromCanada/brew,JCount/brew,JCount/brew,maxim-belkin/brew,DomT4/brew,MikeMcQuaid/brew,mgrimes/brew,Homebrew/brew